Heat treatment method for improving plasticity of TiAl alloy formed by electron beam selective melting

Abstract

The invention provides a heat treatment method for improving plasticity of a TiAl alloy formed by electron beam selective melting. The heat treatment method is characterized by comprising the following steps: analyzing a sedimentary structure; setting hot isostatic pressing parameters; performing hot isostatic pressing state metallographic phase and performance analysis; setting heat treatment parameters; heat treatment state metallographic phase and performance analysis; compared with the grain size and the lamellar cluster proportion in a hot isostatic pressing state, the grain size does not grow abnormally, and the performance is optimal when the lamellar cluster proportion is maximum; after metallographic analysis, testing the mechanical properties of the samples at the temperatures T4, T5 and T6, and comparing the difference of the mechanical properties at the temperatures; and after structure and performance analysis, a final heat treatment system is determined. The method has the advantages that the elongation of the TiAl alloy used for electron beam selective melting forming is improved, the room-temperature elongation of the alloy can be improved by 100% or above, the defect that the room-temperature elongation of the TiAl alloy used for electron beam selective melting forming is low can be effectively overcome, and the application field of the electron beam technology and the TiAl alloy is expanded.

Description

Technical Field

[0001] The invention relates to the field of electron beam selective melting and forming, and in particular to a heat treatment method for improving the plasticity of electron beam selective melting and forming TiAl alloy. Background Art

[0002] TiAl alloy has broad application prospects in the automotive, aviation and aerospace fields due to its high melting point, low density, high elastic modulus, low diffusion density, good structural stability, excellent oxidation resistance and corrosion resistance, and high flame retardancy. TiAl alloy used in aircraft engines is considered to be an ideal material to replace Ni-based high-temperature alloys because it can reduce weight by 20% to 30%. Although TiAl alloy has made breakthroughs in application, its application fields are very few and it is still a long way from large-scale application. The main problem is that the intrinsic brittleness and difficult processing of TiAl alloys have not been fundamentally solved.

[0003] Electron beam melting (EBM) is an important metal additive manufacturing technology. Relying on high-energy beams, local melting is used to achieve discrete-stacking preparation of powders and obtain high-degree-of-freedom sample forming. Compared with traditional forming methods, EBM has unique advantages in preparing room-temperature brittle and difficult-to-process materials. EBM technology uses a high-energy-density, fast-moving electron beam as a heat source. By preheating, the forming temperature is maintained above 900°C, which can effectively prevent the material from cooling rapidly to below the ductile-brittle transition temperature. Therefore, the use of EBM technology to prepare room-temperature brittle TiAl alloys has attracted widespread attention. The dual-structure TiAl alloy low-pressure turbine blades produced by Avio in Italy have a yield strength of 480MPa at 800℃ and good creep resistance. After GE acquired Arcam, a leading EBM technology company, for $1.4 billion in 2016, it also invested heavily in EBM forming research. Its jet engine fuel nozzles produced by 3D printing have been installed on the latest LEAP jet engine. At the same time, the Ti-48Al-2Cr-2Nb alloy blades produced by GE using EBM technology have also entered the installation test stage. At present, one of the main obstacles restricting the practical application of TiAl alloy formed by electron beam selective melting is its poor room temperature plasticity. Summary of the invention

[0004] The purpose of the present invention is to close the defects such as holes and cracks inside the alloy under the lowest temperature conditions at a constant pressure and holding time in the hot isostatic pressing stage, and to inhibit the growth of γ grains; and to transform the γ+α2 matrix into a full lamellar structure under the lowest temperature conditions at a constant holding time in the heat treatment stage. By hot isostatic pressing and heat treatment of the deposited electron beam selective melting TiAl alloy, the final structure after heat treatment is a full lamellar structure, and the room temperature tensile elongation of the electron beam selective melting TiAl alloy is improved, and a heat treatment method for improving the plasticity of the electron beam selective melting TiAl alloy is provided.

[0005] The present invention provides a heat treatment method for improving the plasticity of TiAl alloy formed by electron beam selective melting, which is characterized by comprising the following steps:

[0006] Step 1: Sedimentary structure analysis

[0007] The microstructure of the deposited electron beam selectively melted TiAl alloy samples was analyzed.

[0008] Step 2: Hot isostatic pressing parameter setting

[0009] In the α+γ two-phase region, T1, T2, and T3 were selected respectively, and hot isostatic pressing was carried out under the conditions of holding pressure ≥130 MPa and holding time ≥4 h;

[0010] Step 3: Hot isostatic pressing metallographic and performance analysis

[0011] It is necessary to comprehensively consider the defect closure effect and the influence of mechanical properties to develop a hot isostatic pressing system. The main analysis is whether the pores and cracks are closed, the changes in the organizational morphology, phase volume fraction, and grain size; after metallographic analysis, the mechanical properties of the samples at T1, T2, and T3 temperatures are tested to compare the differences in mechanical properties at each temperature;

[0012] Step 4: Heat treatment parameter setting

[0013] The purpose of this heat treatment process is to further transform the TiAl alloy from a blocky γ+α2 matrix to a full lamellar structure, and further balance and stabilize the structure of the TiAl alloy to improve its elongation; solid solution heat treatment is carried out in the α single phase region, and the heat treatment temperatures are selected as T4, T5 and T6 respectively, and the solid solution time is 4-5h, followed by furnace cooling. The subsequent aging treatment is the same 700-900℃, the time is 8-15h, and then the furnace is cooled to the furnace temperature;

[0014] Step 5: Heat-treated metallographic and performance analysis

[0015] Compared with the grain size and lamellar cluster ratio in the hot isostatic pressing state, the performance is best when the grain size has no abnormal growth and the lamellar cluster ratio is the largest; after metallographic analysis, the mechanical properties of the samples at T4, T5, and T6 temperatures were tested to compare the differences in mechanical properties at each temperature.

[0016] Step 6: Determine the final heat treatment system

[0017] After analyzing the organization and performance, the final heat treatment system is determined.

[0018] Compared with the prior art, the present invention has the following advantages:

[0019] The heat treatment method for improving the plasticity of the electron beam selective melting formed TiAl alloy described in the present invention is used to improve the elongation of the electron beam selective melting formed TiAl alloy. Through optimized hot isostatic pressing and heat treatment processes, the room temperature elongation of the alloy can be finally improved by more than 100%, which can effectively make up for the disadvantage of low room temperature elongation of the electron beam selective melting formed TiAl alloy and expand the application field of electron beam technology and TiAl alloy. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The present invention is further described in detail below in conjunction with the accompanying drawings and embodiments:

[0021] Figure 1 : Hot isostatic pressing + heat treatment process route;

[0022] Figure 2 : Microstructure of TiAl alloy after electron beam selective melting in the as-deposited state; (a) XY direction; (b) Z direction

[0023] Figure 3 :TiAl alloy structure after electron beam selective melting in hot isostatic pressing state; (a) 1245℃; (b) 1280℃; (c) 1290℃

[0024] Figure 4 : Microstructure of electron beam selectively melted TiAl alloy after heat treatment; (a) 1300℃; (b) 1320℃; (c) 1340℃. DETAILED DESCRIPTION

[0025] The present invention will be further explained below in conjunction with specific implementation plans, but the present invention is not limited thereto. The structures, proportions, sizes, etc. illustrated in the drawings of the specification are only used to match the contents disclosed in the specification for people familiar with this technology to understand and read, and are not used to limit the restrictive conditions under which the present invention can be implemented. Therefore, they have no substantive technical significance. Any structural modification, change in proportional relationship or adjustment of size should still fall within the scope of the technical content disclosed by the present invention without affecting the effects and purposes that can be achieved by the present invention.

[0026] Step 1: Sedimentary structure analysis

[0027] The microstructure of the deposited electron beam selectively melted TiAl alloy samples was analyzed. Figure 2 (a) is a cross section, i.e., the XY-axis structure, which is composed of a large number of blocky γ phases, and the grain size of each γ block is ~50μm. Figure 2 (b) is the longitudinal section, i.e., the organization in the printing direction (Z direction), in which there are large-sized γ block cluster grains, as shown by the arrows in the figure.

[0028] Step 2: Hot isostatic pressing parameter setting

[0029] In the α+γ two-phase region, hot isostatic pressing was carried out at 1245°C, 1280°C, and 1290°C, with a holding pressure of 150 MPa and a holding time of 4 h.

[0030] Step 3: Hot isostatic pressing metallographic and performance analysis

[0031] The room temperature tensile properties of the samples under different hot isostatic pressing conditions are shown in Table 1.

[0032] Table 1 Room temperature tensile properties after different hot isostatic pressing treatments

[0033]

[0034] The room temperature tensile properties results show that the tensile strength after hot isostatic pressing at 1245℃ is lower, which is attributed to the unclosed defects; the tensile strength after hot isostatic pressing at 1280℃ is higher, which is attributed to the closure of defects and the insignificant growth of the blocky γ grains; while the tensile strength after hot isostatic pressing at 1290℃ is lower, which is due to the abnormal growth of the blocky γ grains in the structure, resulting in uneven grain size, and the strip-like α2 phase laths formed at the grain boundaries are unfavorable to the strength.

[0035] Therefore, considering the defect closure effect, microstructural characteristics and room temperature tensile properties, the optimal hot isostatic pressing system is 1280℃ / 150MPa / 4h.

[0036] Step 4: Heat treatment parameter setting

[0037] A vacuum resistance furnace was used to heat treat the hot isostatically pressed electron beam selectively melted TiAl alloy samples at different temperatures. Solution heat treatment was carried out in the α single phase region. The heat treatment temperatures were 1305°C, 1325°C and 1345°C, respectively. The solution time was 4 hours, followed by furnace cooling. Subsequent aging treatments were all at the same 900°C for 8 hours, followed by furnace cooling to the furnace outlet temperature. The effects of different heat treatment temperatures on the alloy structure and mechanical properties were studied, and ultimately the optimal heat treatment system was obtained. The microstructures of TiAl alloy samples under different heat treatment process conditions are shown in Figure 2. Figure 3 shown.

[0038] Step 5: Heat-treated metallographic and performance analysis

[0039] When the solution temperature just enters the α phase region, that is, 1305°C, the grain size in the sample remains at a small level, but the proportion of full lamellar layers in the organization is small, such as Figure 4 As shown by the arrow in (a). When the temperature increases to 1325℃, Figure 4 (b), the proportion of lamellar clusters in the structure increases. When the temperature is further increased to 1345℃, the size of the alloy lamellar clusters (including massive γ grains) grows abnormally, such as Figure 4 (c) as shown.

[0040] The room temperature tensile properties of electron beam selectively melted TiAl alloy under different heat treatment process conditions are shown in Table 2.

[0041] Table 2 Room temperature tensile properties after different heat treatments

[0042]

[0043] Step 6: Determine the final heat treatment system

[0044] After the organization and performance analysis, the final heat treatment system was determined to be 1280℃ / 150MPa / 4h, FC+1325℃ / 4h, AC+900℃ / 8h, FC.

[0045] Matters not covered by the present invention are known technologies.

[0046] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A heat treatment method for improving the plasticity of TiAl alloy formed by electron beam selective melting, characterized in that: The following steps are involved: Step 1: Sedimentary structure analysis The microstructure of the deposited electron beam selectively melted TiAl alloy samples was analyzed. Step 2: Hot isostatic pressing parameter setting In the α+γ two-phase region, T1, T2, and T3 were selected respectively, and hot isostatic pressing was carried out under the conditions of holding pressure ≥130 MPa and holding time ≥4 h; Step 3: Hot isostatic pressing metallographic and performance analysis It is necessary to comprehensively consider the defect closure effect and the influence of mechanical properties, and develop a hot isostatic pressing system; analyze whether pores, cracks, etc. are closed, and the changes in organizational morphology, phase volume fraction, and grain size; after metallographic analysis, conduct mechanical property tests on samples at T1, T2, and T3 temperatures, and compare the differences in mechanical properties at each temperature; Step 4: Heat treatment parameter setting The TiAl alloy is transformed from a blocky γ+α2 matrix to a full lamellar structure, and the structure of the TiAl alloy is further balanced and stabilized to improve its elongation; a solid solution heat treatment is performed in the α single phase region, and the heat treatment temperatures are selected as T4, T5 and T6, respectively, and the solid solution time is 4-5h, followed by furnace cooling; the subsequent aging treatment is the same 700-900℃, the time is 8-15h, and then the furnace is cooled to the furnace temperature; Step 5: Heat-treated metallographic and performance analysis Compared with the grain size and lamellar cluster ratio in the hot isostatic pressing state, the performance is best when the grain size does not grow abnormally and the lamellar cluster ratio is the largest; after metallographic analysis, the mechanical properties of the samples at T4, T5, and T6 temperatures were tested to compare the differences in mechanical properties at each temperature; Step 6: Determine the final heat treatment system After analyzing the organization and performance, the final heat treatment system is determined.

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